1. Field of the Invention
The present invention relates to a toner for developing electrostatic images, a charge control agent comprising an amorphous metal complex salt compound of the monoazo series for control or stabilization of the amount of charges of the toner, and a manufacturing process therefor.
2. Description of the Prior Art
In copying machines, printers and other instruments based on electrophotography, various toners containing a coloring agent, a fixing resin and other substances are used to visualize the electrostatic latent image formed on the photoreceptor having a light-sensitive layer containing an organic or inorganic photoconductive substance.
The chargeability of such toners is a key factor in electrostatic latent image-developing systems. Thus, to appropriately control or stabilize the amount of charges of a toner, a charge control agent providing a positive or negative charge is often added to the toner.
Of the conventional charge control agents in actual application, those providing a negative charge for a toner include 1:2 type metal complex salt dyes of monoazo compounds and metal complexes of aromatic hydroxycarboxylic acids such as alkylsalicylic acids.
However, many of the metal complexes having azo dye structure proposed as charge control agents are usually unstable; for example, they are likely to be decomposed or to deteriorate to lose their initial charge control capability when exposed to mechanical friction or impact, temperature or humidity changes, electric impact, light irradiation, etc. Also, even such metal complexes possessing a practically applicable charge providing property are often problematic as to charge stability or often contain impurity chemicals lacking charge control effect due to differences in production method and conditions, posing many problems regarding charge control agent quality stability, reliability and other aspects.
Among charge control agents capable of resolving these problems are the crystalline metal complex salts having the following structures: ##STR1##
In the above formulas, X represents H (i.e. hydrogen), a halogen, --SO.sub.2 NH.sub.2, a nitro group, an alkyl group or the like; and A represents H (i.e. hydrogen), an alkali metal, an amine or the like.
Such metal complex salt dyes are what are called 1:2 type azo metal complex dyes wherein 2 molecules of a monoazo dye are coordinated to 1 trivalent metal atom.
The present inventors found problems to be resolved in such 1:2 type azo metal complex dyes, including the possibility that the metal complex dye, when used as a charge control agent in a toner for a long period of time, can damage the photoreceptor by partially exposed crystals on the surface of toner particles because such dye generally possesses hard crystallinity, and the likelihood that the dye tends to drop (separate out) from toner particles during charging (i.e. frictional charging) due to the hard crystallinity.